feat: extra registration fields improvements

[BryanttV]

Closes: https://github.com/eduNEXT/consulting-issues-mapping/issues/829

Description

This PR improves the FormFieldRenderer component and the configurable registration form with better UX feedback, input validation, and more robust error message handling.

1. Placeholder support for text and textarea fields in FormFieldRenderer.
2. maxLength restriction applied natively on text inputs using fieldData.restrictions.max_length.
3. Animated help text on focus, displays fieldData.instructions while the field is focused, using TransitionReplace.
4. Error message normalization, new normalizeErrorMessage utility that converts backend error messages arriving as objects (e.g., { required: "..." }) to strings, preventing [object Object] from being displayed.

Testing Instructions

Using Tutor:

1. Use changes in this PR: https://github.com/eduNEXT-collab/campus-agora-your-cluster-project/pull/8
2. Create a mount of the MFE, cloning this repository in your tutor root environment, and run tutor mounts add frontend-app-authn
3. Run nvm use && npm install
4. Go to the registration page at http://apps.local.openedx.io:1999/authn/register
5. You should see a new field called Date of birth.
6. You can test the new validations, user feedback, and error messages.

Screenshots/sandbox (optional):

Before	After
Without placeholder and help text	With placeholder and help text
ERROR! ❌	Field required message
Without max length restriction	With max length restriction

[mariajgrimaldi]

This is a very nice improvement to the user experience, but if we don't contribute it back upstream, we'd have to maintain it ourselves. Can we consider opening this against upstream master?

[BryanttV]

@mariajgrimaldi, Yes, I definitely think that would be the best option. I didn't choose to push it directly to upstream right away because the unit tests aren't ready yet, and I'd also like to have it reviewed from a front-end perspective before publishing it to upstream.